Analog Trunks
Because many organizations continue to use analog devices, a requirement to integrate
analog circuits with VoIP or IP telephony networks still exists. To implement a Cisco
voice gateway into an analog trunk environment, the FXS, FXO, DID, and E&M interfaces
are commonly used, as illustrated in Figure 3-19.
152 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
FXO
Port
FXO
Port
DID
Port
CO
PSTN
PSTN
Station Port
FXS Interface FXO Interface
DID Interface
Trunk Side of PBX
E&M Interface
CO
V
V
FXS
Port
FXS
Port
FXS
Port
V
E&M Port
V
Figure 3-19 Analog Trunks
PSTN carriers typically offer analog trunk features that can be supported on home
phones. Table 3-5 presents a description of the common analog trunk features.
Table 3-5 Analog Trunk Features
Feature Description
Caller ID Caller ID allows users to see the calling number before answering
the phone.
Message waiting Two methods activate an analog message indicator:
■ High-DC voltage message-waiting indicator (MWI) light and
frequency-shift keying (FSK) messaging.
■ Stuttered dial tone for phones without a visual indicator.
Call waiting When a user is on a call and a new call comes in, the user hears an
audible tone and can “click over” to the new caller.
Caller ID on call waiting When a user is on a call, the name of the second caller is
announced or the caller ID is shown.
Table 3-5 Analog Trunk Features (continued)
Feature Description
Transfer This feature includes both blind and supervised transfers using the
standard established by Bellcore laboratories. The flash hook
method is common with analog trunks.
Conference Conference calls are initiated from an analog phone using flash
hook or feature access codes.
Speed dial A user can set up keys for commonly dialed numbers and dial
these numbers directly from an analog phone.
Call forward all Calls can be forwarded to a number within the dial plan.
Redial A simple last-number redial can be activated from analog phones.
DID Supported on E&M and FXS DID ports.
Figure 3-20 shows small business voice networks connected through a gateway to the
PSTN. The voice network supports both analog phones and IP phones. The connection to
the PSTN is through an FXO port, and the analog phone is connected to the small business
network through an FXS port. The issue in this scenario is how the caller ID is
passed to call destinations.
Chapter 3: Routing Calls over Analog Voice Ports 153
PSTN
Caller ID Display
Number 408 555-0100
Name ACME Enterprises
Caller ID Display
Number 555-0112
Name John Smith
Analog Extension
Station ID Number 555-0112
Station ID Name John Smith
Call 1
Call 2
Service Provider Database
Number 408 555-0100
Name ACME Enterprises
Ext. 0113
408 555-9999
V
Figure 3-20 Analog Trunks - Example
This example describes two calls; the first call is to an on-premises destination, and the
second call is to an off-premises destination:
■ Call 1: Call 1 is from the analog phone to another phone on the premises. The FXS
port is configured with a station ID name and station ID number. The name is John
Smith, and the number is 555-0212. When a call is placed from the analog phone to
another phone on the premises, an IP phone in this case, the caller name and number
are displayed on the screen of the IP phone.
■ Call 2: Call 2 is placed from the same analog phone, but the destination is off the
premises on the PSTN. The FXO port forwards the station-ID name and station-ID
number to the CO switch. The CO switch discards the station ID name and station
ID number and replaces them with information it has configured for this connection.
For inbound calls, the caller ID feature is supported on the FXO port in the gateway. If
the gateway is configured for H.323, the caller ID is displayed on the IP phones and on
the analog phones (if supported).
154 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
Note Although the gateway supports the caller ID feature, Cisco Unified
Communications Manager does not support this feature on FXO ports if the gateway is
configured for Media Gateway Control Protocol (MGCP).